The invention relates to unsaturated homo- and/or copolymerizable linear polyesters, prepared from diols and at least one unsaturated dicarboxylic acid or its derivatives capable of forming esters; a method for manufacturing same and their use.
Homo- and/or copolymerizable unsaturated polyesters, are known compounds as are methods for their manufacture and processing. Thus, unsaturated polyester resins can be produced by copolyesterification of mono- and/or multivalent alcohols with mono- and/or multivalent carboxylic acids and/or, optionally, hydroxy-carboxylic acids containing alcoholic hydroxyl groups or also using or co-using their derivatives capable of forming esters. In the process, at least one compound which is olefinically unsaturated and homo- or copolymerizable, is condensed into the polyester resin where the average functionality and the ratio of hydroxyl to carboxyl groups must be chosen according to rules well known in the art so that the formation of polyesters is assured. These resins can then be hardened into thermo-setting plastics also in thick layers particularly after the addition of radical-providing compounds and, optionally, after the addition of olefinically unsaturated copolymerizable monomers.
These known polyesters are used particularly in the form of their solutions in copolymerizable vinyl and/or allyl monomers as casting resins for impregnating electric machinery or parts thereof. They are useful to impregnate, for instance, stators of electric motors or transformers by different impregnating methods such as immersion or sprinkling. Furthermore, electrical or electronic components or complete circuits can be superficially coated with them.
After the immersion of the machinery or components, they are conducted through an evaporation and dripping zone and then the adhered polyester resin is hardened in an oven. With this method, dripping losses first occur in the dripping zone. Even though the resin accumulating here can be returned to the immersion tank, there is the disadvantage that the dripping leads to an incomplete impregnation. Furthermore, additional dripping losses occur in the oven prior to the hardening if the viscosity of the casting resins drops due to the increase in temperature. This is particularly disadvantageous because the material dripping off in the oven hardens and cannot be reused. It also leads to considerable contamination of the oven.
A further problem which results from the dripping-off or running-down of the casting resins is the occurrence of nonuniform layer thicknesses which increase from top to bottom, and the development of beads at the lower edge of coated components.
To avoid these disadvantages, it is known to generate structural viscosity or thixotropy in the casting resins by adding to them substances which produce flow anomalies. For this purpose, substances such as pyrogenic silica, asbestos fibers, quartz meal, microdolomite and liquid components such as castor oil derivatives, acrylic resins or silicone resins are customarily used. The use of these additives gives rise to several disadvantages. The inorganic components do not yield clear solutions and it is not always simple to work them into the polyester resins. The above-mentioned liquid components are disadvantageously present in the resins as softeners. Further, a skin can develop on the casting resins if these additives are used.
It is an object of the invention to avoid these disadvantages and to create polyesters which exhibit useful flow anomalies due to their molecular structure and which exhibit structural viscosity or thixotropic behavior without the addition of supplementary additives.